Adsorptive heat transformation arrangements are known in the art. There are thus sorption heat pumps which are used for heating buildings and which use higher-temperature heat provided by a gas burner. Further, sorption refrigerators are also known, which are used primarily for cooling, it preferably being possible to use heat of a temperature of less than 100° C. from solar collectors. In both cases, the physical principle of adsorption and desorption is being used, wherein a temperature range is used, at which adsorption heat in a system can be dissipated. The differential heat curves of adsorption and desorption give the maximum recoverable heat within the cycle. Internal heat recovery reduces the desorption heat to be provided from the external heat source and the efficiency of the cycle increases.
WO 2008 034 561 A2 describes an adsorption heat pump and refrigerator which comprises a thermally stratified heat store, which is used primarily for internal heat recovery between an adsorption and desorption half-cycle of a heat pump or refrigerator and which comprises a single adsorber. As a result, the efficiency of a cycle, in other words the coefficient of performance of the heat pump, can be substantially increased by comparison with a cycle without heat recovery. This stratified store receives the heat transfer fluid and can store it in various layers depending on the temperature thereof. Heat can thus be taken from the stratified store at different temperatures and stored again. In the case of use as a heat pump, a particularly large overlap of the adsorption and desorption heat curves, and thus a particularly high coefficient of performance of the heat pump, can be achieved using adsorbents requiring desorption temperatures greater than 140° C. However, in this case a heat transfer oil is required as the heat transfer fluid and storage medium, since if water were used the store would have to be configured for a high pressure and would have to be treated as a pressurised container under the EU Pressure Equipment Directive. However, in the heating technology sector, the use of heat transfer oil is associated with very high market entry barriers, and so an adsorption heat pump of this type is only suitable to a limited extent as a gas heat pump for heating buildings. If adsorbents are used which can be desorbed at lower temperatures, water can indeed be used as the heat transfer and storage fluid, but the achievable efficiency is thus reduced.
In general, the stratification in the heat store is provided for example by stratified charge lances having a large number of openings arranged vertically within the heat store. Because of the density and pressure differences occurring in the fluid of a different temperature, the fluid virtually stratifies itself when it exits the openings. However, at high flow speeds, there is increased mixing of the adjacent layers, meaning that a continuous temperature transition is produced rather than a “discrete” layer.
Because of the high specific heat capacity, the good heat exchanging properties and negligible costs, water is a preferred storage medium in many applications, including in heating technology. However, if desorption temperatures of approximately 130° C. are used, the difficulty occurs that the saturation steam pressure of the water exceeds an acceptable pressure for which major system components, in particular in the heating circuit, are configured. This pressure is typically 3 bars. In heat stores in particular, the steam pressure must not exceed the system pressure so as to prevent spontaneous steam formation in the store. This could for example result in pressure surges in various system components and damage them.
EP 1 985 948 A1 describes an adsorption heat pump which describes temporarily hydraulically decoupling part of the fluid circuit. However, this circuit is at a very high temperature (for example 140° C.) at the end of the desorption cycle, and this temperature has to be reduced, so as to avoid a pressure surge, before a valve for the heating circuit can be opened. A valve arrangement is used for the temporary hydraulic decoupling. A circulation pump is switched on for a certain period of follow-up time so as to distribute the heat of the hot adsorber and thus cool the adsorber. The pressure in this circuit can thus be reduced to the level of the heating water pressure. This has the drawback that a long follow-up time is required for cooling the sorption material, and so the process of pressure reduction in the decoupled part of the fluid circuit takes a very long time and the power of the heat pump is reduced.
U.S. Pat. No. 4,509,337 A1 discloses a solar-thermal-powered zeolite adsorption refrigerator which comprises a temperature-stratified heat store. The appliance is operated in a day/night cycle, the store primarily serving as a buffer store for short phases without solar radiation. However, the arrangement of the store does not make it possible to recover adsorption heat for the subsequent desorption.
DE 10 2012 024 073 A1 describes a stratified heat store comprising stratified apparatuses for flow abatement. Here, very fine actuation of said stratified apparatuses is required, since high specific volume flows of the adsorbers occur between adjacent stratified apparatuses.
DE 10 2011 102 036 B4 discloses a heat recovery system for an adsorption heat pump having an intermediate store as the heat store. This intermediate store is used to store sensible heat which is released or required when the temperature in an evaporator/condenser component changes. In a first operating phase, the heat store displays a temperature gradient which extends in the opposite direction to a temperature gradient of a second operating phase inside the store. Thus inaccurate use of the heat store as a stratified store is disadvantageous.
Further, multi-stage adsorption heat pumps are known which contain a plurality of adsorbers having different adsorbents, adsorption heat from an adsorber being used for desorbing another adsorber. One possible arrangement for implementing a process of this type is described in the dissertation of T. Nunez (“Characterising and evaluating adsorbents for heat transformation applications”, Faculty of Physics, Freiburg University, 2001). Using multi-stage adsorption processes of this type, very high coefficients of performance of the heat pump can be achieved; the drawback of the designs known thus far is the very high equipment outlay for implementing the process.
DE 199 08 666 A1 describes an adsorption machine having two adsorbers in a “thermal wave” arrangement, wherein a heat exchanger is dedicated to each adsorber and functions as an evaporator in an adsorption half-cycle and as a condenser in a desorption half-cycle. A stratified store is used to recover heat in this case. Each heat exchanger is connected to the adsorber assigned thereto by means of a channel for the adsorbate, which is a vaporous operating agent.
DE 34 08 193 C2 further describes a multi-stage hybrid sorption pump which functions as a resorption heat pump having an adsorption stage as the high-temperature part. Two adsorbers are provided, which are connected to two storage tanks, and solutions of the adsorption stage of the hybrid heat pump which contain either a large or small amount of operating agent.